When it comes to the musical mainstream, Sigur Rós – a post-rock group from Iceland, known for their impenetrable lyrics and atmospheric, celestial sound – is pretty out there. But over the past few years, their music has been slowly worming its way into our ears, heard in the films of Wes Anderson and on television shows like CSI. More recently, the band has headlined at Madison Square Garden, performed on Late Night with Jimmy Fallon and The Tonight Show with Jay Leno, and even appeared as themselves on that ultimate benchmark of pop-culture relevancy: an episode of The Simpsons.

No matter how seemingly uncommercial their sound, after nearly 20 years playing together, bassist George Hólm says, “I think we’ve kind of established ourselves as the underground band that everyone knows of and everyone listens to.” It’s that kind of quiet, but steady infiltration that has led Sigur Rós, which formed in 1994 in Reykjavik, to surpass Bjork as the biggest act to come out of Iceland, with multiple world tours under their belt and music videos on YouTube that have combined views in the tens of millions.

Not that success has tempered the band’s drive to experiment and explore, as evidenced on their new album, Kvieker. Now working as a trio after the departure of keyboard player Kjartan Sveinsson in 2012, the remaining members – singer Jónsi Birgisson, drummer Orri Páll Dýrason and Hólm – have embraced a new, darker sound. Gone almost completely are the sweeping piano chords found on previous albums; in their absence, the album has a distinctly rougher, grittier sound. “It’s more aggressive,” admits Hólm, though he points out it has “its mellow and sweeter side.”

But it’s the darker side that kicks off the album, with the opening track “Brenninsteinn” blending a distorted bass and heavy percussion. The track does taper off, mellowing into a dream-like sound that has become Sigur Rós’s signature, but the mood has been set for a more hard-hitting album. Similarly, Kvieker’s title track and second single has a grainy, distorted anthem-like pulse that’s as unnerving as it is enthralling. This album – the group’s seventh – seems set out to prove the band’s long-stated, but mostly unheard, love of heavy metal.

Yet fans will find tracks like “Hrafntinna,” with its ethereal, rolling cymbals, and “Iskjaki,” with Birgisson’s distinctive falsetto, familiarly comforting. Also recognizable to fans will be the group’s peculiar lyrics. As on past albums, Birgisson uses a mixture of Icelandic and made-up words, a strange and beguiling argot they call Hopelandic. Fans still request direct translations, but this “language” has no real vocabulary.

“Jónsi’s not really singing any words; he’s just singing syllables, just babble. He kind of writes the vocal line in that way,” says Hólm. “We’re quite lazy writing lyrics. The music comes first and then when everything’s finished and we’re almost completely done with the record, we find the time to sit down and think about the lyrics. It’s probably pretty dumb but it’s how we work for some reason.”

Not that the band does much sitting around. They’re currently in the middle of another world tour that includes stops across the U.S., Europe and Asia, but Hólm says they’re looking forward to taking a break from touring after this year. “We’d love to do some projects we’ve been talking about for years, which we wanted to do but were never able to because we never had any time. We were either in the studio recording a record or touring.” What sort of projects? “Maybe to score a movie or something,” he says.

It seems that, for Sigur Rós, not playing their music is the truly out-there thing to do.